I figure you've got it (#25) since you posted this, but i figured I'd help anyone else looking for it since i just FINALLY (1.5 hr) found it today. In the first room of the area with the plants, go up the stairs right in front of you. Turn left, go all the way to the room at the end. To your right is a box of ammo in front of a window. The watcher is on the other side of the window at about eye level.